1. Field of the Invention
The present invention relates to a liquid crystal display device having a driving method in which the polarities of signal potentials are inverted when they are written to the pixel electrodes.
2. Description of the Related Art
A driving method of a liquid crystal display device that is commonly employed conventionally will be described with reference to FIGS. 10-12.
FIG. 10 shows a liquid crystal display device system and polarities of display signals that are written to a display panel 1 of the system. Reference numerals 2 and 3 denote a signal line driver and a scanning line driver, respectively. Reference numeral 4 denotes a display timing controller for controlling the signal line driver 2 and the scanning line driver 3. FIG. 11 shows, in the forms of two-dimensional pictures, (n-1)th-field and nth-field display signals in FIG. 10. In the picture of the (n-1)th field, plus signals and minus signals are written alternately from the first line to the nth line. In the next, nth field, the polarity of the lth-line signal is changed from plus to minus.
FIGS. 12A and 12B show an output signal of the signal line driver 2 and output signals of the scanning line drivers 3 in the nth field, respectively. In FIG. 12A, signals shown above the chain line are display signals of positive polarity and those shown below the chain line are display signals of negative polarity. As shown in FIG. 12A, negative-polarity (minus) signals are output continuously for the lth line for which the polarity is changed from the (n-1)th field and for the lines before and after it, that is, the (1-1)th and (1+1)th lines. Therefore, for the lth line and the (1+1)th line, the display signals have no rising portion from the lowest potential; that is, they have no voltage transient period. Therefore, the amplitude of the final potential in those scanning lines is larger than that in other scanning lines by .DELTA.Vsig [V]. As a result, in the lth line and the (l +1)th line, display signals having a larger amplitude by .DELTA.Vsig [V] are written and hence brighter display is effected than in other scanning lines. The increase of .DELTA.Vsig [V] in signal amplitude results from the driving ability of the signal line driver 2, particularly its output impedance.
FIG. 12A also shows, by a broken line, an output signal of an ideal signal line driver whose output impedance is zero. In this case where the output impedance is zero, the polarity inversion of the signal is completed instantaneously and the final value of the signal amplitude is kept constant irrespective of whether portions of the same polarity exist; there occurs no problem in display. However, actually, since the driver 2 has a certain, non-zero output impedance value, a low-pass filter is formed by a parasitic capacitance in the liquid crystal panel 1 and the output impedance of the driver 2, which causes a problem that waveform distortion occurs as indicated by the continuous line and a flicker occurs on the display screen.